Debugging GLSL

From a past post:

There are only two debugging techniques in the universe:

1. printf.
2. /* */

Is that true when writing GLSL shaders? Yep. Commenting out things is natively available. What about printf? The GLSL equivalent of printf is

gl_FragColor.rgba = vec4(stuff_i_want_to_see,...);

That is, you simply output an intermediate product to the final color, run your shader, then view something else. This is how I debug some of the more complex shaders: I view each product in series to confirm that my intermediate values aren't broken. Since the sim is running at 30 fps, I can move the camera and confirm that the values stay sane through a range of values.

The numeric output is often not in a visible range - to get around that I often use a mix of abs, fract (to see just the lowest bits), scaling, and normalize() to sanitize the output.

One app feature is critical: make sure you can reload your shaders in a heart-beat. In X-Plane we have a hidden menu command to do this. This way, you can move your printf, recompile the shaders, and see the change.

A visual debugger is a useful tool for debugging C/C++ because you don't have to commit to what you will view before compiling - you can just print any intermediate product from the debugger. For GLSL, make the recompile cycle fast, and you'll be able to simply edit the code in near-realtime.